1.    Introduction
The foot, which has a complex and multi-joint structure, has the greatest effect on the lower body during dynamic movements such as running (3). In this regard, the measurement of foot pressure distribution among the methods of running analysis is a traditional and new method that examines the performance of the foot in static and dynamic conditions (3). By examining the pressure distribution of the sole of the foot, biofeedback rehabilitation can be used to control the posture of walking and running. And useful information can be obtained regarding the structure and function of the foot and the general mechanics of walking and running (5). The maximum pressure of the sole of the foot is used as the maximum load on different areas of the foot during the support phase of walking and running (6). In order to diagnose foot problems, design soles, analyze sports performance, prevent injury and improve balance control (7), information related to the distribution of pressure on the sole of the foot is used. Any change in the pressure patterns of the sole of the foot increases the possibility of tissue damage and pain (8). Despite the importance of measuring plantar pressure, therefore, by reviewing the history of research, little research has been done on the field of plantar pressure in athletes, and so far no research has been done on athletes in the field of athletics.
Although many studies have investigated the effect of structural factors such as age, body mass index, or the use of different devices in measuring pressure, despite the efforts of researchers, there are very few reports about the distribution of plantar pressure in the field. There are sports, especially in athletics, while no research was found that investigated the effect of increasing speed on foot pressure in athletics, so the information related to the standard range of pressure due to the lack of studies in this field is not specifically available. Therefore, the aim of the current study is to determine the effect of increasing running speed on plantar pressure in athletics athletes.
2.    Methods
In order to carry out the present applied research, which was conducted in a semi-experimental manner, 28 of the elite runners of Tehran province who were in one of the country's athletics league teams voluntarily participated as a sample in the present research. Subjects ran on a treadmill while kinematic data were collected through a 3D imaging system with 12 cameras. The research protocol included the subjects first walking at a speed of 1.2 meters per second to familiarize themselves with the treadmill. Then, slowly, the speed reached 2.5 m/s and after 3 minutes of running at this speed, the data was recorded in about 30 seconds. The runners ran on the treadmill at a speed of 3.5 m/s and then at a speed of 4.5 m/s. And after 3 minutes of running at any speed for 30 seconds, data was collected.
Finally, for the statistical analyses of the data, Kalmogorov Smirnov test, analysis of variance with repeated measures and Bonferroni post hoc test were run using SPSS version 22 software at a significance level of 0.05.
3.    Results
The results of the analysis of variance with repeated measurements showed that increasing the running speed has a significant effect on the displacement and the speed of the center of pressure of the soles of the runners. Also, the results of the follow-up test showed that between running at a speed of 2.5 m/s and running at a speed of 3.5 m/s (p=0.001), between running at a speed of 2.5 m/s and running at a speed of 4.5 m/s (p=0.000), and between running at a speed of 3.5 m/s and running at a speed of 4.5 m/s (p=0.001), there is a significant difference in the displacement of runners. As the running speed increases, the amount of displacement and the pressure speed of the soles of the feet also increases.
4.    Conclusion
The results indicated the effect of increasing the running speed on the amount of displacement and the speed of the plantar pressure center in runners. In the explanation of the present finding, it can be stated that any change in the pattern of walking and running affects the distribution of plantar pressure (14). It should be noted that with an increase in running speed, the length of the stride may increase or the frequency may increase, which can be due to the fact that a longer stride length causes an increase in the forces applied to the floor of the foot (15). This may be one of the reasons for increasing the speed of the plantar pressure center. In this regard, Peter and Theodor (2014) concluded that there is a direct relationship between impact speed and plantar pressure (9).
Among other possible mechanisms related to foot pressure, we can mention the effect of fatigue on foot pressure, so that in the condition of fatigue, the reduction of muscle control in the joints of the foot and ankle may lead to the ability to resist torsion disrupting the dynamic form. Even some of the previous studies have related the coefficient of changes of peak pressure to the process of re-controlling the balance (8). In the present study, despite the fact that running did not reach the point of exhaustion, fatigue occurred sooner with the increase in speed and time of running. This may also be another reason for the findings of the present research.
On the other hand, it can be pointed out that when running and increasing the speed, the support phase is reduced and the swing phase is increased. The maximum value of the support phase in running is 20 percent, and this increase and decrease in running can reduce electromyography activity and ground reaction forces (16). But this point should be mentioned that as the time of support decreases while running, the reaction forces of the ground also decrease, which can be accompanied by a decrease in the loading rate during running (17), so it is possible to increase the pressure of the sole of the foot. At the same time, speed increases can be due to the peak contact force of the ground reaction force. However, more research is needed in this field in order to definitely find the most important factors in this field.


Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be considered in this research.
Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors.
Authors' contributions
All authors equally contributed to preparing article.
Conflicts of interest
The authors declared no conflict of interest.